Originally Posted by
bobthib
The human body is designed to use glucose first before fat. So there is no problem with what Chaco and BarracksSi are proposing. If you are burning 750 - 1000 cals per hour, taking on some carbs is obviously not going to present a problem.
However, if you are in keytosis, and you have been there for you several weeks, your metabolism has changed to efficiently burn fat. You start with very little glucose in your muscles, but you won't bonk. Fat is a better fuel for long sustained efforts. What it is not good for would be a final all out sprint effort.
From my reading, the jury is still out on that, though I freely admit I'm not a scientist. Friel, one of the authors of
The Paleo Diet for Athletes, found that a pure low-carb diet wasn't working for the athletes he trained who did long distance events, i.e. over 2 hours. Even though his athletes had been on a low carb diet for a long time, they still couldn't convert their fat quickly enough to sustain performance. So the authors modified the traditional low carb diet to add some carbs, but only during the event or immediately after. They did not recommend carb loading meals before the event or hours after. I'm grossly oversimplifying their reasoning, but that's the gist of it.